Thiol-terminated sulfur-containing compounds are known to be well-suited for use in various applications, such as aerospace sealant compositions, due, in large part, to their fuel-resistant nature upon cross-linking. Other desirable properties for aerospace sealant compositions include low temperature flexibility, short curing time (the time required to reach a predetermined strength) and elevated-temperature resistance, among others. Sealant compositions exhibiting at least some of these characteristics and containing thiol-terminated sulfur-containing compounds are described in, for example, U.S. Pat. Nos. 2,466,963, 4,366,307, 4,609,762, 5,225,472, 5,912,319, 5,959,071, 6,172,179, 6,232,401, 6,372,849 and 6,509,418.
Another important characteristic for aerospace sealant compositions is strong adhesion to typical aircraft substrates, such as substrates used in fuel tanks. Exemplary materials for such substrates include metals, such as aluminum and aluminum alloys. As a result, aerospace sealant compositions often include adhesion promoting ingredients, such as epoxy, thiol, or amino functional silanes, which are known to bond an organic composition, such as a sealant composition, to an inorganic substrate, such as certain metals, via a hydrolyzable group that forms a metal-siloxane bond (Si—O-metal).
Thus, sealant compositions that are storage stable but, when applied to a substrate, can be cured quickly to form a cured sealant having the characteristics described above are desired. Compositions that cure when exposed to actinic radiation, such as ultraviolet radiation, as can be the case with the reaction of thiol functional compounds with ethylenically unsaturated compounds (sometimes referred to as “enes”), are a candidate for such sealant compositions. Such compositions may include a photoinitator that generates free radicals upon exposure to ultraviolet radiation. These free radicals lead to crosslinking via a thiol-ene reaction, which can often be completed within seconds.
It is believed that one drawback to the use of such compositions is that the rate of the crosslinking reaction can be so fast that conventional adhesion promoting agents, such as the epoxy, thiol, or amino functional silanes mentioned earlier, do not have sufficient time to react with the substrate prior to cure of the sealant, thereby reducing their effectiveness. As a result, it is desirable to provide sealants exhibiting good adhesion to aircraft substrates, such as fuel tank substrates, even when formed as a result of a very rapid radiation curing mechanism.